libstdc++: Enable std::text_encoding for darwin and FreeBSD

The <xlocale.h> header is needed for newlocale and locale_t on these
targets.

libstdc++-v3/ChangeLog:

	* acinclude.m4 (GLIBCXX_CHECK_TEXT_ENCODING): Use <xlocale.h> if
	needed for newlocale.
	* configure: Regenerate.
	* src/c++26/text_encoding.cc: Use <xlocale.h>.

Reviewed-by: Iain Sandoe <iain@sandoe.co.uk>